1. Field of the Invention
The invention relates to a touch probe, which is used on a coordinate positioning machine (for example a coordinate measuring machine or machine tool), to determine the position of a surface.
Such a probe comprises a fixed structure provided by a housing, by which the probe may be supported on the movable arm (e.g. a quill or spindle) of the machine. A stylus supporting member is supported in the housing in a rest position from which the supporting member may be displaced under the influence of a deflecting force, and to which the supporting member may return when the deflecting force is removed. The purpose of the supporting member is to carry a stylus. In use the machine is operated to drive the stylus into contact with a surface; the probe detects contact between the stylus and the surface, and causes a signal to be sent to the machine control which instructs the machine (a) to record the position of the movable arm at this instant, and (b) to brake movement of the movable arm. The ability of the supporting member to move from its rest position accommodates the small movement (known as overtravel) of the movable arm (and thus the probe) which occurs after the stylus has contacted the workpiece, and before the movable arm may be brought to a complete standstill. Such a touch probe may be oriented in many different ways in order to perform a measuring operation, and in order to ensure that the supporting member remains in its rest position when the stylus is not in contact with a workpiece, biasing means are provided to bias the supporting member into its rest position. The biasing means also serve to ensure that the supporting member will return to its rest position after the measuring operation in which the stylus (and thus the supporting member) has been deflected.
2. Description of Related Art
Traditionally, the biasing means has a low spring rate, to provide a small rate of increase of biasing force with displacement of the supporting member. This ensures that for large displacements of the supporting member, the biasing force will not be too large (a large biasing force can damage the stylus). The biasing means is pre-loaded to provide a sufficiently large biasing force on the supporting member when the member is in its rest position.
It is often the case that different lengths of styli will be supported on the supporting member, and that as a consequence, the force applied by the biasing means to the supporting member will need to be adjusted. A known mechanism for adjusting the biasing force on the supporting member is shown in EP 390,342, in which a spring bears against the supporting member at one end, and terminates at the other end in a cylindrical shuttle which is constrained to move within a bore provided on the fixed structure. The biasing force applied by the spring is adjusted by altering the position of the shuttle within the bore and thus the degree of compression of the spring.